This invention relates to sensors for sensing the level of a dielectric liquid and more particularly to such sensors for sensing the level of oil, transmission fluid or the like under normal and extreme temperature conditions.
It is desirable in many situations to detect the actual level of a dielectric liquid in a range of possible levels, as opposed to detecting the presence or absence of the dielectric liquid at one predetermined level. Such situations for example include the detection of level of fuel in a fuel tank for an internal combustion engine. Other such dielectric liquids include engine oil and transmission fluid, both also commonly used in connection with internal combustion engines. It should be appreciated that the detection of the actual level of certain dielectric liquids such as oil and transmission fluid is far more difficult than the detection of the level of engine fuel due to the more extreme environmental conditions encountered in engines and transmissions.
Sensors for detecting the actual level of oil and transmission fluid must be capable of accurately sensing the level of oil and transmission fluid over a wide range of temperatures. Moreover, this wide range of temperatures can be encountered with a few minutes during starting and operation of an internal combustion engine. Temperatures for such sensors can easily go from ambient up to 150.degree. C. in engines and to 200.degree. C. in transmissions. Clearly fuel level sensors to do not normally encounter these temperature extremes. Another unique feature involved in measurement of the level of dielectric fluids such as oil and transmission fluid is the fact that the level itself changes substantially once the internal combustion engine is started. While the engine is at rest, the oil or transmission fluid pools in the lowest parts of their respective blocks and housings. However, once the engine is started the oil is sent throughout the engine block and the transmission fluid is dispersed throughout its housing. During running conditions, it is not as important to known the actual level of the oil or transmission fluid, but it is vitally important to known whether there has been a catastrophic loss of such oil or fluid. Present sensors are not believed to provide both high accuracy during the period before starting, and adequate warning of catastrophic loss, in a sensor of reasonable size and cost.
Various oil level sensors are currently used in automotive and related applications to supply oil level information to engine control systems, including computerized control systems. Such information is particularly important in the case of heavy equipment such as mining and other heavy construction equipment. The working environment of the engine and transmissions in these cases is particularly harsh and the equipment is expensive. Although the present invention is especially suited to such environments, it is not so limited.
Some sensors which are currently in use to sense the oil level in internal combustion engines are known as thermal dissipation sensors. These sensors are relatively low in cost and are relatively easy to interface with control systems, but they do have certain features which could be improved. For example, the power consumption of thermal dissipation sensors is undesirably high. In addition, these sensors tend to cause the amount of carbon in the oil to increase, which is not desirable. Furthermore, when such sensors are used in transmissions to detect the level of the transmission fluid, they are adversely affected by the spray present in a transmission whenever it is running.
Float switches are also used for oil level sensing and these switches are relatively low in cost. However, float switches are not particularly reliable and they suffer from a limited operating temperature range. Mechanical or optical sensor such as dip sticks and sight glasses have also been used as oil level and transmission fluid level sensors, but these latter sensors are relatively inconvenient to use and they do not interface well with electronic or computerized control systems. Their utility is further reduced because of the fact that they must in general be read manually.
A capacitive probe sensor is also available for sensing oil level. It has the advantage of being a low power consuming device, but it does suffer from a reduced operating margin. That is, present capacitive probes are not believed to adequately discriminate between the minute capacitance changes occuring as oil levels change, and the other error-causing effects which occur due to the extended range of operating conditions of the engine itself.
Capacitive sensors suffer from the fact that a relatively low capacitance is being measured (which is a result of the limited space available). Moreover, the electronics for present capacitive probe sensor are believed to be too expensive for widespread use in low-cost applications such as automotive applications. This low capacitance and the cost considerations require that any interface circuit between the sensor and a control system be built as part of the sensor itself. The accuracy of present capacitive sensors is also believed to be affected by the fact that the dielectric constant of the oil or transmission fluid is a function of temperature so that the capacitance measured by the sensor will vary as the temperature of the oil or transmission fluid varies.
In addition, the conductance of various oils and transmission fluids also varies with temperature in a way that affects the output of capacitive sensors. This conductivity variation makes necessary either special circuitry for detecting the imaginary component of the output to eliminate errors caused by conductivity or special constant current charging of the capacitive sensor. Both approaches introduce undesirable cost. In addition, in the latter approach, errors are introduced due to the switching required to charge and discharge the capacitors. This makes it difficult to obtain good stability within a large operating temperature range, particularly when small capacitance values are involved.
Also, at high temperatures, various semiconductor components such as diodes and transistors which one would normally incorporate in an interface circuit have relatively large leakage currents which provide another source of error. In addition, stray capacitances introduce errors, as does the use of discrete components in the detection circuitry (due to minor mismatches in the responses of the discrete components with variations in temperature and with the passage of time). It is also the case that the dielectric constants of various oils and transmission fluids vary from oil to oil and fluid to fluid so that the output of a capacitive sensor representing the supposed level of the oil or transmission fluid could vary depending the particular oil or transmission fluid present.